Semiconductor quantum rods provide color tunable and polarized emission. As such, they are highly relevant building blocks for applications in flat panel displays in various approaches. These and other characteristics of the quantum rods, such as their wet chemical processing, and charge separation ability, also provide their relevance for additional applications exploiting fluorescence and other beneficial properties of the quantum rods. A particular architecture of interest is the seeded rod, where a seed, for example of a quantum dot of CdSe or ZnSe, is used to seed the growth of a rod [1-3]. The resultant dot-in-rod structure is often referred to as combining OD-1 D characteristics in one nanostructure.
The synthesis of semiconductor rods by seeded growth methodologies is based on a high temperature injection of cadmium chalcogenide quantum dot and sulfur precursor to a flask containing cadmium precursor and several ligands, including phosphines, phosphine oxides and phosphonic acids [2,4]. This yields highly monodisperse nanorods with controlled length and width. The combination of high band gap CdS nanorod with lower band gap CdSe seeds results in a type I or quasi type II band alignment and a significant fluorescence quantum yield [5,6].
The current synthesis, however, is so far limited to seeded CdS nanorods alone [1,7]. When an additional ZnS shell was grown over CdSe/CdS nanorod [8], the quantum yield of the nanorods increased somewhat but by a limited extent, exemplifying that variation in the rod composition could change and improve its properties. Such approaches also proved useful in more standard core/shell quantum nanoparticles compositions [9,10]. However, the aforementioned synthetic approach of the second overcoat on the seeded rods, used a combination of highly reactive chemicals with complex synthesis approach, and the resulting quantum yields and stability performance were also not maximal.
Therefore, there is an emerging need to expand this seeded rods architecture to other types of semiconductors, particularly to high band gap semiconductors or their multicomponent compounds. In particular, there is need to expand the seeded grown rods family also to Zn-chalcogenides containing rods materials, which have proven to be a challenge not addressed successfully yet, due to the synthetic difficulties. Successful addressing of this challenge can provide increased fluorescence quantum yields, increased stabilities to the seeded rods, along with further ability to tune the emission color for the desired application; in particular addressing shorter wavelength emission. The incorporation and growth of Zn-chalcogenide rods in seeded growth approach is also desirable from the viewpoint of reducing Cd content in the materials, as required by various environmental concerns.
In several works the composition of CdSe/CdS semiconductors was attempted to be modified using post synthesis cation exchange process [11-13]. Ability to replace Cd+2 with Cu+2, Zn+2, Pb+2 or Ag+2 in the rod was demonstrated, resulting in a Cu2Se/Cu2S, ZnSe/ZnS, PbSe/PbS and Ag2Se/Ag2S nanorods, respectively. The cation exchange was performed using a two-step process: first Cd+2→Cu+2, followed by Cu+2→Zn+2, Pb+2 or Ag+2. However, this approach has several limitations. This approach utilized additional processing steps which complicated the synthesis. The quantum yield of the nanorods produced has decreased rather than increased, and no versatility in the control of the nanorod composition has been shown.
In another work [12], partial cation exchange was achieved, resulting in a segmented CdSe/CdS/ZnS like-structure. This is also most likely related to the process approach, and hence the utility of such rods is of limited use for applications such as in displays requiring high quantum yield and stability.
Therefore, there remains the need to develop seeded nanorods with controlled composition, in particular nanorods with significant content of Zn chalcogenides, with controlled composition and enhanced properties.